The present invention relates generally to paint baking ovens for use in the automotive industry and, in particular, to a paint baking oven with radiant tube heating elements.
Paint baking ovens, especially in the automotive industry, are well known. Paint baking ovens are utilized to cure the surface of a freshly painted product, such as an automobile body, in an enclosed, dust free environment. A typical prior art paint baking oven is constructed of an insulated enclosure that is heated and through which the painted vehicles are conveyed. Prior art ovens typically have two sections, a first or xe2x80x9cbring upxe2x80x9d section, and a second or xe2x80x9choldxe2x80x9d section. Each section of the oven may have several zones. The first section of the prior art oven uses a flat panel or curved panels that are heated from behind the panels to provide radiant heat to heat the vehicle in a calm, dust free atmosphere. The second section of the prior art oven uses convection to maintain the cure temperature for a predetermined cure time. Each zone of a conventional oven uses a large exterior heater to supply the heat for curing the paint. In addition, heated fresh air is introduced into the oven for safety ventilation of the fumes released during the curing process. The heater is an insulated box having a gas-fired burner and fans disposed therein to recirculate the air via conduits or ducts in the oven. The heater can be of a direct fired or indirect fired type and may or may not include filters, depending on the type of application. The gas-fired burners of the prior art are known as modulating burners that cycle from high fire to low fire, and vary the amount of fuel to the burner to increase or decrease the temperature supplied to the oven zones.
The instrumentation utilized for measuring the cure temperature on the vehicle surface has improved over the years and can now measure multiple points within the oven body. The temperature data recorded from prior art paint baking ovens indicates that the conventional prior art paint baking ovens do not provide a uniform cure temperature.
Radiant tube heaters are a well known type of heater used in paint baking ovens. Radiant tube heaters typically include an inlet and an outlet and a reflector member attached at an outer periphery thereof for directing the radiant heat in a desired direction. Combustion products are introduced at the inlet and flow toward the outlet, transferring heat to the walls of the tube while flowing from the inlet toward the outlet. The walls of the tube then emit radiant heat to the interior of the oven body. Radiant tube heaters, however, have been used only for providing supplemental heat for the vehicle body rocker panels in prior art automotive paint bake ovens. The prior art radiant tube heaters disadvantageously provided poor temperature control and tended to overheat the thinner metal on the vehicle in part because the prior art radiant tube heaters required the burners to be mounted adjacent to the tube. The radiant tube heaters were limited in length because of the tube being straight and because of the difficulty of mounting burner components inside the oven enclosure. Due to their limited length, radiant tube heaters have only been used in the initial portion of the bring-up section in prior art paint baking ovens.
Radiant tube heaters, however, have been used in other applications such as heat treating furnaces where the temperature over the length of the tube was not difficult to control because of the higher temperatures at which the furnace operates. Radiant tube heaters are advantageous for indirect heat because they operate using less excess combustion air, and therefore use less energy while providing the same or greater amount of heat to the oven.
It is desirable, therefore, to provide a paint baking oven having uniform cure temperatures throughout the length of the oven. It is also desirable to provide a more efficient paint baking oven utilizing radiant tubes heaters as a heat source throughout the entire length of the oven.
The present invention concerns an apparatus for curing paint on a surface of a vehicle body. The apparatus includes an oven enclosure having an oven section for receiving the vehicle body. A plurality of radiant tube heating elements is disposed in the oven section and each of the radiant heating tube elements has an exterior surface. An oven heater includes at least one burner disposed therein and is positioned external to the oven section. The oven heater is connected to the heating elements for providing heat energy to the oven section. A control means is connected to the oven heater for turning the oven burner on and off to maintain a predetermined temperature at the exterior surfaces of the heating elements whereby uncured paint on a surface of a vehicle body in the oven section is cured.
The paint baking oven according to the present invention uses at least one radiant tube heater that is heated with a pulse-fired gas burner to provide a uniform surface temperature on the radiant tube. The timing of the pulse-fired gas burner is controlled by the control means, which monitors the surface temperature of the radiant tube. When the surface temperature of the radiant tube drops below a predetermined value, the burner is ignited and when the surface temperature of the radiant tube exceeds another predetermined value, the burner is extinguished. A continuous spark in the gas burner is used to ignite the gas and air mixture. The gas and air is then mixed with a small portion of excess air, allowing the combustion of the gas and air mixture at near stoichiometric conditions. The radiant tube heaters are preferably constructed of standard materials and will be used to transfer heat to vehicles with wet paint that is to be cured. The burner is advantageously mounted external to the oven enclosure, which allows the use of radiant tube heaters throughout the entire length of the oven.
The paint baking oven according to the present invention includes a xe2x80x9cbring upxe2x80x9d section, a xe2x80x9ctemperature stabilizationxe2x80x9d section, and a xe2x80x9choldxe2x80x9d section. Each of the sections includes a plurality of zones therein. The vehicle body having a coat of fresh paint is introduced into the xe2x80x9cbring upxe2x80x9d section and later moves to the xe2x80x9ctemperature stablizationxe2x80x9d section and finally to the xe2x80x9choldxe2x80x9d section. The vehicle body is conveyed through the oven by a conveyor or the like and remains in each zone for a predetermined time interval.
The paint baking oven according to the present invention advantageously does not require the large exterior heater for heating recirculated air used on prior art conventional ovens but does utilize the heater for heating ventilation air. The present invention also does not require flame supervision or recirculation air filters as in the prior art. The present invention includes a paint baking method that provides a uniform cure temperature and a paint baking oven that can be installed with less capital cost and can operate with less energy than prior art paint baking ovens, thereby reducing operation cost. The present invention also provides an oven that has a straightforward design and is easy to operate and occupies less vertical space than a conventional prior art oven.